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I'm attempting to wire in a 12 V fan to my car that draws 7 A at full speed. The fan is a fused link directly to the battery. I have a 12 V 20 mA signal wire, but that doesn't appear to be strong enough to trigger a standard automotive relay RL44.
enter image description here

Are there any options to support such a high switching current from such a low signal current?


@Winny and @Spephro Pefhany suggested a pilot/bootstrap. Is this the idea of how that would work?

schematic

simulate this circuit – Schematic created using CircuitLab

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    \$\begingroup\$ Bootstrap yourself with a transistor, preferably MOSFET to pull the relay? \$\endgroup\$ – winny Sep 15 '18 at 21:43
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    \$\begingroup\$ Follow the link above and replace your bottom relay with a MOSFET. \$\endgroup\$ – winny Sep 16 '18 at 10:45
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    \$\begingroup\$ Choose a MOSFET with low enough RdsON to not cause excessive heating by P=0.14^2*RdsON. Don’t forget a flyback diode across your relay coil. Or just use a BJT and resistor as in this one: i.stack.imgur.com/FVgTy.png \$\endgroup\$ – winny Sep 16 '18 at 11:13
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    \$\begingroup\$ @Bill, when you use the CircuitLab button on the editor toolbar you can save an editable version of your schematic in your question. No account needed and no need for screengrabs with the grid showing. We can also copy your schematic and edit it in our answers. \$\endgroup\$ – Transistor Sep 16 '18 at 12:27
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    \$\begingroup\$ You need a pulldown resistor on the input, and some ESD protection as FET gates are quite sensitive to ESD. \$\endgroup\$ – peufeu Sep 16 '18 at 12:29
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You could use a small pilot relay or a transistor to switch the automotive relay.

An example of such as relay would be the G5Q-1A4 DC12 from Omron, about 1.50 USD each. It takes 16.7mA coil current.

It's rated at 10A current so it may be able to switch the fan directly, but the automotive relay may last longer particularly if you don't put a diode across the fan. DC rating is 10A carry and 5A switched, so it's marginal and the initial surge may cause the contacts to weld so probably not a good idea.

An RL44 is rated at 40A (resistive load) and the coil draws 140mA.


Edit:

schematic

simulate this circuit – Schematic created using CircuitLab

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    \$\begingroup\$ It looks like that G5Q-1A4 supports 10 A for AC, but is only 5 A DC. Good find on the RL44 coil draw... i've been searching and couldn't find it. \$\endgroup\$ – Bill Tarbell Sep 15 '18 at 22:27
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    \$\begingroup\$ This is true, rated to carry but not switch 10A. Another reason to use the automotive relay to switch the fan itself is that it will be more resistant to welding from the initial surge with the motor stalled. \$\endgroup\$ – Spehro Pefhany Sep 15 '18 at 22:33
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    \$\begingroup\$ Not enough information- is the "signal" active high or low? Is it open when off or actively pulled in each direction? There is enough information to say your schematic is wrong and will blow up (diode is the wrong way and the MOSFET is the wrong type). Also MOSFET gates need some protection in an automotive environment. I didn't suggest the MOSFET, because I thought it would get complicated compared to just a relay. @Winny can answer that if he wants. Added schematic for pilot relay with coil diode, assuming active high. If that's wrong you can flip it or use the NC contact. \$\endgroup\$ – Spehro Pefhany Sep 16 '18 at 12:43
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    \$\begingroup\$ A 1N400x (eg. 1N4005) can be put across the RL44 coil too (same direction as the pilot relay). \$\endgroup\$ – Spehro Pefhany Sep 16 '18 at 12:50
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    \$\begingroup\$ Thank you. My experience is minimal, so i appreciate simplicity. \$\endgroup\$ – Bill Tarbell Sep 16 '18 at 12:51

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